Touch interface devices can include computing devices having touch sensitive surfaces used to receive input from operators of the devices. For example, many smart phones, tablet computers, and other devices have touch sensitive surfaces that identify touches from operators as input to the devices.
Some of these devices have smooth touch surfaces with an approximately constant friction across the entire surface. Some other known devices have the ability to change the friction forces experienced by a fingertip on the surface. The friction forces may be controllably reduced by introducing ultrasonic vibrations perpendicular to the plane of the surface. The vibrations may be mechanically generated using ultrasound transducers (e.g., piezoelectric elements). A limitation of these devices is that friction is a resistive force, meaning that the friction force opposes the motion of the fingertip. Yet other known devices may apply shear forces to the fingertip in a variety of directions, and not just in directions that oppose motion of the fingertip. These devices may generate the forces by synchronizing in-plane vibrations of the touch surface with the gating (e.g., switching) on and off of ultrasonic vibrations that control the magnitude of the friction. Due to the time required to gate on and off ultrasonic vibrations, however, the frequencies at which the in-plane vibrations occur may be limited.
Moreover, mechanically producing the vibrations can generate acoustic noise that can be undesirable. Additionally, the extent of variation of frictional forces that are achievable by the use of mechanical vibrations may be limited.
The shear forces supplied by some of these known devices may be constant or approximately constant across the entire touch surface at any moment of time. For example, these devices may be incapable of providing different shear forces on different fingertips that concurrently or simultaneously touch the same surface of the device.